Break testing

Discussion in 'Search, Rescue and Technical Skills' started by Jeff Randall, Jun 28, 2018.

  1. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    During our recent adventure through Heaps Canyon we managed to core shot a 8.3mm rope on the final rappel. After we got finished we wanted to break test the rope to see what it would really hold even though it looked pretty nasty. After finishing fabrication on our break test machine, I tested the damaged rope. It broke at 8.28 kN (1861 Lbs). The surprising thing is it did not break where the core was exposed. Instead it broke about 8 inches below that in a spot that did not appear to be damaged.

    Test-Machine.JPG
    Test machine I fabricated.

    Core Shot.JPG
    Rope we damaged in Heaps Canyon and had to rappel on for the last rap.

    Setup.JPG
    Setup for break testing.

    Break.JPG
    Break at 8.28 kN. Note it did not break in the damaged area.

    IMG_2750.JPG
    Dynamometer we are using on our break test machine.

    IMG_2754.JPG
    RAT carabiner broke at 25 kN

    IMG_2755.JPG
    Doing some accessory cord testing.
     
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  2. Strigidae

    Strigidae Moderator Staff Member

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    This is cool. I would have bet money it would have broken at the damaged part. Any idea why it did not? Can you test that rope again to see if it will break there?
     
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  3. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    There was obviously some damage that was not noticeable, thus making that the weakest point. I doubt I have enough to test again without using some type of rope grab and that will most likely just de-sheath the remaining portion before it broke.
     
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  4. Strigidae

    Strigidae Moderator Staff Member

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    Makes me wonder if i can ever trust a rope again. I guess at 8 kn id have to really be doing the wrong thing.
     
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  5. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    Ha! Makes me trust them even more.
     
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  6. Strigidae

    Strigidae Moderator Staff Member

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    What was the published breaking point?
     
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  7. DYSPHORIC JOY

    DYSPHORIC JOY Moderator Staff Member

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    Excellent post.
     
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  8. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    New rope is 4100 lbs.
     
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  9. Strigidae

    Strigidae Moderator Staff Member

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    Ok well after panic reaction that sounds much better than i thought.
     
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  10. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    Still almost a 10:1 safety factor. Granted I wouldn't want to use it but we had no choice on that final rap. It was either use it or stand on the bird perch and wait for rescue.
     
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  11. shaneadams90

    shaneadams90 ESEE Knives Marketing Director Staff Member

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    Just thinking about all the stuff that is gonna get "Tested" now makes me smile a bit....

    Aren't our RAT carabiners rated to 24kn?
     
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  12. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    That was the whole purpose of building this break test machine. As much as we use ropes and hardware, I like to break things every so often just to see how things are holding up, not to mention gather more intel for our own work.

    Yes, the carabiners are rated at 24 kN, but even with 3 Sigma testing it still means that a few out of a 1000 can break below 24 kN. 99.73% will break at or above the 3 Sigma rating.

    And in case you're interested, here's the process for 3 Sigma: :)

    1. Test a sampling of parts. Write down the breaking strength of each.

    2. Work out the Mean (average) number (Breaking strength of each divided by number of parts tested)

    3. Then for each breaking strength number: subtract the Mean and square the result

    4. Then work out the average of those squared differences.

    5. Take the square root of that average to find standard deviation

    6. Multiply the standard deviation by three

    7. Subtract this from the average breaking strength (Mean) of all tested parts. Result is the 3 Sigma MBS rating. It is 99.73% probable any additional breaks will be at or above this final value.
     
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  13. Trailmaster

    Trailmaster Member

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    I got lost at step 4
     
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  14. Mudman

    Mudman Member

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    There's no way to replicate the core shot Jeff? This has really peaked my interest.
     
  15. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    Yes, we could probably get close on replicating it but that still would not tell us anything about this particular section of that rope. Obviously on this section there was a place that was a weaker point than the core shot itself. If we replicated the core shot on another section of rope then did another test, it would most likely break in the core shot.
     
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  16. McKROB

    McKROB Member

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    What was the mechanism of the damage?

    Not rope, but I’ve had 54 conductor seismic borehole cable get jammed in a borehole, I applied a lot of force (sustained and jerk) over several days and when I finally got it unstuck the wires (analogous to the rope core) were broken internally in the order of tens of meters from where the sheathing was broken (binding point). Breakage was found using a reflectometer and 100m of the cable had to be thrown out. Even where the wire was broken the insulation around it was not. Makes me wonder if you had a similar sort of thing happen with your rope, I suspect it was damaged while under load?
     
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  17. Jeff Randall

    Jeff Randall ESEE Knives / Randall's Adventure & Training Staff Member

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    This is what I suspect happened. The exposed core shot came from rub on a sharp edge but I suspect the real culprit was due to something else
     
  18. Bushman5

    Bushman5 Member

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    i love stuff like this. Really interesting to see that break point in the rope!
     
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  19. Montanan

    Montanan Member

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    Here's a core shot I witnessed happen. Would have been fun to test that rope after.

    [​IMG]
     
  20. OKcherokee

    OKcherokee Member

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    And here I was certain that after I escaped college statistics with a D I’d never be exposed to it again.
     

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